An example of means for obtaining three-dimensional shape information on an object by image analysis includes a method of analyzing a deformation amount of an optical pattern changed according to a three-dimensional shape of a target object by projecting the optical pattern onto the target object existing in a predetermined imaging visual field. A light-section method, a spatial encoding method, a fringe analysis method can be cited as typical examples. These methods are based on the principle of triangulation. Among others, for the fringe analysis method, there are proposed many techniques such as spatial fringe analysis and temporal fringe analysis, and the fringe analysis method is known as a technique in which high measurement precision is obtained.
In these techniques, usually an area sensor is used to read the target object onto which the optical pattern is projected. However, when the area sensor is used, because the target object does not fit into one imaging visual field, it is necessary that the area sensor be moved in both a lengthwise direction and a crosswise direction to take the one imaging visual field while the one imaging visual field is divided into plural portions, which results in the imaging time being lengthened.
Patent Documents 1 and 2 propose a three-dimensional shape measuring method in which a line sensor is used instead of the area sensor. Especially, the measuring method disclosed in Patent Document 1 is performed by the following steps. The target object onto which the optical pattern is projected is taken by a line camera, subsequently the phase of the projected optical pattern is shifted by moving the target object, and the target object onto which the optical pattern having shifted phase is projected is taken by another line camera; the above processes are repeated plural times. The optical patterns included in the plural taken images are analyzed to measure the three-dimensional shape based on the temporal fringe analysis method (phase shift method).
In a three-dimensional shape measuring apparatus proposed in Patent Document 2, the three-dimensional shape is measured by the following method. The target object onto which the optical pattern is projected is taken separately by two line cameras placed at different positions, and the three-dimensional shape is measured from the two taken images based on the stereo method.
Patent Document 1 proposes the three-dimensional shape measuring method in which a line sensor is used instead of the area sensor. The measuring method disclosed in Patent Document 1 is performed by the following steps. The target object onto which the optical pattern is projected is taken by the line camera, subsequently the phase of the projected optical pattern is shifted by moving the target object, and the target object onto which the optical pattern having shifted phase is projected is taken by another line camera; the above processes are repeated plural times. The optical patterns included in the plural taken images are analyzed to measure the three-dimensional shape based on the temporal fringe analysis method (phase shift method).
In the three-dimensional shape measuring apparatus proposed in Patent Document 2, the three-dimensional shape is measured by the following method. The target object onto which the optical pattern is projected is taken separately by two line cameras placed at different positions, and the three-dimensional shape is measured from the two taken images based on the stereo method.
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-286433 (published on Oct. 3, 2002)
Patent Document 2: Japanese Unexamined Patent Publication No. 2004-117186 (published on Apr. 15, 2004)
In the conventional techniques, however, there is difficulty in arranging the line sensor or it takes a long time to take the image.
For example, the temporal fringe analysis method of taking the image of the same portion of the target object plural times from the same angle while changing the phase of the optical pattern projected onto the target object is used as the fringe analysis method in the technique described in Patent Document 1. In this case, it is necessary that plural line sensors be used and that all the line sensors be strictly arranged in parallel in order to taken the image of the same portion of the target object conveyed in a linear direction. It is also necessary that all the line sensors be arranged at the same distance from a reference plane on which the target object is placed. Additionally, it is necessary that all the line sensors be arranged in the same attitude in order to taken the image from the same angle. For example, four line sensors are used in Patent Document 1. However, it is actually difficult for the four line sensors to be arranged as described above.
Thus, in the technique described in Patent Document 1, arrangement of the line sensor becomes difficult. In addition, because the plural line sensors are required, upsizing, price increase, and the increased frequency of troubles might arise in the measuring apparatus.
In using the temporal fringe analysis method, there is also a method of taking the image of the target object plural times with one line sensor instead of the plural line sensors. In this case, because parallel processing cannot be performed, the amount of time for taking the image necessary for analysis becomes several times (for example, four times when the four images are taken) compared with the case where the plural line sensors are used. Therefore, the time necessary to the measurement is lengthened.
On the other hand, in the technique described in Patent Document 2, a three-dimensional shape is measured by the stereo method from the two images obtained by the two line sensors, and it is necessary to correctly know the geometric arrangement of the two line sensors to measure the three-dimensional shape by the stereo method. Therefore, in this case, it is also necessary that the two line sensors be correctly arranged, which results in the arrangement of the line sensor becoming difficult. Furthermore, since the method of the Patent Document 2 uses the stereo method, it is difficult to improve the measurement precision of the three-dimensional shape to a resolution not more than a pixel of the line sensor. Therefore, the measurement precision decreases.